Conventionally, many types of foam have been used as a seat cushion to form the entire body of the cushion. Polyurethane foam has been so used and has provided excellent cushioning. However, when such foams are used for the entire cushion, local transformation of the bearing surface often generates fatigue in the user over a lengthy period. In addition, insufficient surface strength in relation to the applied localized strength reduces the durability of such cushions.
To correct such defects, systems have been proposed wherein reinforcing materials have been incorporated into the seat. For example, the seating surface of a foam cushion has been reinforced by a sheet produced by heat-melting one face of a foam sheet. This technique is disclosed in Japanese Patent Application laid open No. 57-772. A different system for reinforcement is shown in Japanese Patent Application laid open No. 61-170360, which discloses embedding an oriented plastic filament in the surface of a polyurethane foam object.
Still another earlier method of reinforcement is disclosed in Japanese Patent Application laid open No. 61-234806. There, films or sheets are interposed into a foam cushion, the interposed films or sheets having a different hardness than the foam. Specifically, this patent application shows a composite cushion comprising a lamination of cushions having 25% compression hardness (JIS K6401) of greater than 15 kG/200 mm diameter and those having that of less than 14 kG/200 mm diameter with the interposition of sheets into them.
Still other cushions have been formed from polyurethane foam differing in hardness on the top and bottom bearing surfaces and both sides. In this system, which is disclosed in Japanese Patent Application laid open No. 61-137732, the cushion is combined with matted filament impregnated with binder.
Still other prior methods of reinforcement, as described in Japanese Patent Application laid open No. 59-80212, involved the combination of foam with metal spring components or by tensioning filament materials onto a frame. However, such constructions are not entirely satisfactory, from a variety of performance and manufacturing aspects.
As noted above, previous foam reinforcement with filaments or sheet materials have provided ordinal warp and weft reinforcement of the same degree. In other words, the resulting construction was a sheet having great rigidity, with no synergistic spring function.
As discussed above, cushions prepared entirely from soft foam often result in a sense of fatigue in the seat occupant. Even combinations of low and high density foams or various reinforcing components have not been satisfactory. Moreover, formation of a cushion entirely from foam with high density makes it impossible to obtain a favorable soft sense of contact. Cushions having fabric reinforcement are inferior to spring suspension systems.
A need therefore exists for reinforcement material having a spring function that provides improvement in the performance of soft foam cushions such as those used for chairs and bedding, and which provides a desirable resilience or bounce in the vertical direction. Another continuing need to which the present invention is directed is the provision of cushioning materials which have reduced thickness without the bottoming that so often accompanies a decrease in thickness.